If a friend came up to you and told you he knows when you’re sleeping and when you’re not, you’d probably think he spends his time sitting across the street from your bedroom window with binoculars and a box of donuts, but the truth is far scarier. Now someone doesn’t need to physically stalk you (or visually monitor you in any way) to know when you’re sleeping. They simply have to download this source code from GitHub, which allows any of your friends to compile a profile of your sleeping habits from your Facebook usage data.

The code was created by Danish software developer Søren Louv-Jansen after conceptualizing just what was possible to extract from the seemingly insignificant data trails we all leave when interacting with social media sites. "By creating a simple service that checks Facebook every 10 minutes, I’m able to get an accurate picture of my friends’ Facebook usage," Louv-Jansen wrote on Medium. "Many people visit Facebook as the first thing in the morning, and the last thing before going to bed. It is therefore possible to get a good impression of their sleeping habits (or lack thereof)."

Specifically, Louv-Jansen says the data he scrapes which he then compiles into an estimate of a friend’s sleeping habits comes from the web-based Messenger tool that Facebook offers that allows people to chat online. That tool logs when a user was last active on the site. Louv-Jansen says he didn’t create the code just because he could. Louv-Jansen told the Washington Post he did it to demonstrate how important online privacy is and because he wants people to "be aware that they’re leaving some digital footsteps everywhere they go."

"Everybody I’ve shown this to have been equally fascinated and outraged by the accuracy with which it predicts their sleep habits," Louv-Jansen wrote on Medium. "In this digital world we leave footprints where we go, and when we do it, without even thinking about it. Facebook might block this little ‘hack,’ so your friends no longer can track you, but Facebook will always be able to do their own data analysis which is undoubtedly way better than what I’ve come up with. They are likely using this data for profiling, and creating more user-specific ads."

Louv-Jansen told the Post that Facebook contacted him to tell him his code violated its terms of service and they requested he discourage others from using it. So far, he’s refused to remove the code from GitHub.

"People should be aware whatever they do, they’re not alone, someone is always watching," Louv-Jansen told the Post. "I don’t want to say that Facebook is evil. This is just a side effect of what they’re doing."